1. Field of the Invention
This invention relates to a surge protection device for protecting electric and electronic circuits from abnormally high voltages and currents caused by, for example, lightning, switching surges or the like.
2. Description of the Prior Art
A wide range of devices referred to as surge protection devices have been devised. Even the number of such devices that fall in the category of two-terminal surge protectors is considerable. The better of these are not limited to the function of clamping the voltage across the device terminals at a fixed breakdown voltage at the time of breakdown caused by occurrence of a surge (i.e. do not function simply as constant-voltage diodes). Instead they further exhibit negative characteristics when the device current that begins to flow at the time of the device breakdown increases to above the breakover current value. As a result, the voltage across the terminals after breakdown is shifted to a clamp voltage that is lower than the breakdown voltage. It therefore becomes possible to absorb large currents.
Among the surge protection devices of this type some utilize an avalanche or Zener breakdown mechanism, while others use the punch-through breakdown mechanism. The present inventor previously developed a number of improvements in the punch-through type surge protection device. These are described in detail in Japanese Patent Application Public Disclosure Nos. Sho 61-237374, 61-259501, 62-65383 and 62-54776 and Japanese Patent Application Publication No. Hei 1-33951.
FIG. 7 shows an example of the sectional structure of a surge protection device 10 according to the prior art.
The surge protection device, designated by reference numeral 10, has a first semiconductor region 1 which is of either p or n conductivity type (n in the illustrated example) and is capable of serving as a semiconductor substrate. A second semiconductor region 2 and a third semiconductor region 3 are successively formed on one principal surface of the semiconductor region 1 by double diffusion. A fourth semiconductor region 4 is further formed on the other principal surface of the semiconductor region 1 as opposed to the second semiconductor region 2.
In the following explanation, the surface formed with the second semiconductor region 2 and third semiconductor region 3 will be referred to as the "front surface" and the surface opposite thereto as the "back surface". Moreover, the various semiconductor regions will be referred to simply as "regions."
Structurally speaking the surge protection device 10 shown in FIG. 7 is formed with the regions 1, 2, 3 and 4 stacked vertically in the thickness direction of the region 1. Moreover, as will be clear from the explanation of the device's operation given later, the device current resulting from surge absorption flows mainly in the thickness direction of the first region, between the third and fourth regions. The device can therefore be said to be of the vertical type. In contrast, there are also prior art surge protection devices of the lateral type in which the fourth region 4 is situated on the front surface at a position offset laterally from the second and third regions 2, 3 and the device current during operation also flows laterally. The present invention however relates only to an improvement in the vertical type.
The second region 2 and fourth region 4 have to be of opposite conductivity type from the first region 1 so that each makes a pn junction therewith. Therefore, as shown in the figures, in the case where the first region 1 is of n conductivity type, the second region 2 and the fourth region 4 are of P conductivity type. In the case where punch-through is utilized as the initial breakdown phenomenon in the manner to explained later, however, it is preferable to constitute the second region 2 to be of somewhat low concentration p conductivity type, namely to be of p.sup.- type.
On the other hand, the third region 3 and the first region 1 have to be of the same conductivity type as each other and of opposite polarity type from the second region 2 so as to form carrier injection junctions for injecting minority carriers into the second region. Since, as will be explained later, the third region 3 constitutes one end of the main device current path after breakdown, it preferably has high conductivity (i.e. is preferably of n.sup.+ type).
At the surface of the fourth region 4 and the surface region 11 of the region 1 to the side of the fourth region 4 is provided a first device terminal T.sub.1 (the lower side terminal in the figure) which is in ohmic contact with both of the regions 4 and 11. In addition, a second device terminal T.sub.2 is provided in ohmic contact with the surfaces of the second region 2 and the third region 3.
For explaining the most basic operation of the illustrated surge protection device 10 during surge absorption it will first be assumed that the device terminal T.sub.1 is in ohmic contact only with the surface of the fourth region 4.
When a surge voltage arises across the first and second device terminals T.sub.1, T.sub.2 at a relatively large magnitude and in such phase as to apply a reverse bias across the pn junction between the first region 1 and the second region 2, the depletion layer produced at the pn junction between the first and second regions by the application of the reverse bias grows not only inwardly into the first region 1 but also toward the third region 3. When the upper extremity of this depletion layer reaches the third region 3, a punch-through state is established between the first region 1 and the third region 3.
When this happens, minority carriers (from the viewpoint of the first region 1) are injected into the first region 1 from the fourth region 4. Since the injected minority carriers collect in the second region 2, device current begins to flow. The voltage at which this punch-through operation starts is designated as breakdown voltage V.sub.BR in the operating characteristics of these surge protection devices shown in FIG. 8. On the other hand, even if the second region 2 and the third region 3 should be shorted at their surfaces by mutual connection with the second device terminal T.sub.2, once the device current begins to flow via the second region 2, as indicated in FIG. 8 by the portion of the characteristic curve designated as a, where the curve rises rapidly in the direction on the current axis, and rises to the point that the product between itself and the resistance along the path thereof in the second region 2 becomes equal to the forward voltage at the pn junction formed between the second region 2 and the third region 3, the pn junction turns on so that minority carriers (from the viewpoint of the second region 2) are injected from the third region 3 into the second region. This injection of minority carriers into the second region 2 causes the device current flowing between the first and second device terminals T.sub.1, T.sub.2 to become even larger. Since this in turn promotes the injection of minority carriers from the fourth region 4 into the first region 1, a positive feedback is obtained.
Thus, as can be seen from the voltage vs current characteristic curve in FIG. 8, when the current flowing between the first and second device terminals T.sub.1, T.sub.2 becomes greater than the value indicated as the breakover current I.sub.BO, the positive feedback within the device clearly manifests itself in the form of the negative resistance characteristic segment b of the characteristic curve. As a result, the voltage across the first and second terminals T.sub.1, T.sub.2 shifts to a clamp voltage (or ON voltage) that is lower than the breakover voltage V.sub.BO at which breakover commenced and also lower than the breakdown voltage V.sub.BR at which punch-through first started. Therefore, as shown by the segment of the characteristic curve marked c, the device is able to absorb large surge currents while holding down the amount of heat it generates.
The maximum current which the surge protection device 10 can absorb across its first and second terminals T.sub.1, T.sub.2 is generally referred to as its "surge absorption capacity" I.sub.pp. On the other hand, the minimum device current capable of maintaining the device in its on state after it has once turned on is called its "hold current" I.sub.H. Differently from the breakover type surge protection device, in the case of, for example, the simple constant-voltage diode type surge protection device, the voltage across the terminals does not fall even after breakdown but, to the contrary, tends to rise gradually with increasing current absorption. Because of this, the device power consumption, i.e. the voltage across the terminals multiplied by the device current, and in consequence the amount of heat generated by the device become considerably large. The superiority of breakover type devices is obvious from this alone.
While the basic operation is as described above, the following additional function is realized as a result of the fact that the first device terminal T.sub.1 is in ohmic contact with not only the surface of the fourth region 4 but also the surface region 11 of the region 1 located to the side of the fourth region 4.
As explained earlier, in the surge protection device shown in FIG. 7, the first region 1 and the second region 2 form a pn junction that is reverse biased during occurrence of a surge. A capacitance C.sub.j can therefore be assumed to be present at this junction.
Presuming that the first terminal T.sub.1 is not in contact with the first region 1, the occurrence of a surge with a through-rate of dV/dt across the first and second device terminals T.sub.1, T.sub.2 would lead to the flow of a displacement current i.sub.t as expressed by Eq. 1 for charging the junction capacitance C.sub.j. EQU i.sub.t =(dV/dt)C.sub.j ( 1)
When the areas of the respective regions are increased for obtaining a large surge absorption capacity, the junction capacity C.sub.j also generally becomes large, a value of, say, 100 pF or larger not being unusual. On the other hand, extensive in-depth studies and research conducted up to now have provided detailed information about the characteristics and behavior of surges. It is thus known that a lightning-induced surge on a telephone line, for example, may well result in a through-rate (dV/dt) of as high as around 100 V.mu.s even in cases where the peak induced noise voltage in the circuit is low.
By substituting this value into Eq. 1, it is clear that the value i.sub.t of the displacement current for charging the junction capacitance may reach around 10 mA. Moreover, since it increases in proportion to the through-rate, it can be seen that the displacement current i.sub.t can reach quite a high instantaneous value.
Thus the device breaks over under a surge whose peak voltage does not reach the breakover voltage V.sub.B0 prescribed by the device design, namely under a noise that would not ordinarily need to be absorbed, simply because the noise happened to be a particularly sharp one with a very high dV/dt. (Such a surge will hereinafter be referred to as a "small surge.") What this means in terms of the characteristic curve of FIG. 8 is that the effective breakover voltage V.sub.BO at the time such a misoperation occurs is shifted to a smaller value than that indicated by the curve.
In contrast to the foregoing, when the first device terminal T.sub.1 is also in ohmic contact with the surface region 11 of the region 1 near the fourth region 4, a surge of a polarity which reverse biases the first region 1 and the second region 2 is applied. Therefore, the junction between the first region 1 and the fourth region 4 is forward biased. In this state, in the early phase of the surge voltage rise before the forward biased junction turns on, majority carriers (from the viewpoint of the first region 1) are injected into the first region 1 from the terminal T.sub.1 side. As a result, the junction capacitance C.sub.j of the pn junction formed between the first region 1 and the second region 2 is rapidly charged. This technique has in fact been employed for realizing a surge protection device which does not respond to small surges. Moreover, the initial injection of majority carriers into the first region 1 for charging the junction capacitance proceeds without adversely affecting the aforesaid basic operation occurring after the start of breakdown. The reason for this is that when the increase in the majority carrier current following punch-through of the first region 1 and the third region 3 causes the voltage drop on the majority carrier current path along the fourth region 4 in the first region to become equal to the forward voltage across the junction between the fourth region 4 and the first region 1, minority carriers (from the viewpoint of the first region 1) begin to flow into the first region 1 from the fourth region 4 so that, from this time on, the device makes the transition from breakdown to breakover in accordance with the mechanism described earlier.
While the foregoing explanation was made with respect to a conventional punch-through type device, the inventor has also found that by increasing the thickness of the second region 2 and, in addition, appropriately selecting the geometric, impurity concentration and other parameters of the respective regions, it is possible to fabricate a surge protection device which uses avalanche breakdown or Zener breakdown as the initial breakdown mechanism for realizing a turn-on operating mechanism which is similar to that of the aforesaid punch-through type device as far as breakover is concerned.
As will be understood from the foregoing, the provision of the device terminal T.sub.1 in ohmic contact with not only the fourth region 4 but also the surface of the first region 1 has enabled conventional surge protection devices to achieve a certain degree of freedom from misoperation under exposure to small surges. The problem is, however, that a surge protection device 10 fabricated in this manner is unable to provide an adequately large reverse withstand voltage. This is because the first region 1 is in direct contact with the device terminal T.sub.1 via the ohmic contact portion 11 in a state where the first region 1 and the second region 2 are in a forward biased relationship and, therefore, the device terminals T.sub.1, T.sub.2 are in essence simply connected through a forward biased diode. The reverse direction characteristics of the device are therefore as indicated by the curve d in FIG. 8.
The conventional technology is therefore incapable of enabling fabrication of a surge protection device that is able to absorb surge irrespective of whether the positive surge polarity appears on the first device terminal T.sub.1 or on the second device terminal T.sub.2. If the first terminal T.sub.1 should not have the portion 11 in ohmic contact with the first region 1, it would be possible to absorb surge of either polarity by forming the fourth region 4 in the same manner as the second region 2 and forming a fifth region in the fourth region 4 in the same manner as the third region 3 is formed in the second region 2. In this case, at the occurrence of a reverse polarity surge (i.e. a surge of the polarity causing the second device terminal T.sub.2 to become positive), the pn junction at which punch-through (or avalanche or Zenner breakdown) occurs would be the second pn junction formed by the first region 1 and the fourth region 4 and the function previously played by the third region 3 would be taken over by the fifth region. The negative characteristics obtained with this arrangement would thus enable absorption of reverse polarity surges.
If, however, a second device terminal T.sub.2 should be provided in ohmic contact with both the surface of the second region 2 and the surface of the first region 1 for charging the junction capacitance of the second pn junction between the fourth region 4 and the first region 1, just as the first device terminal T.sub.1 was provided in ohmic contact with both the surface of the fourth region 4 and the surface of the first region 1 for charging the junction capacity of the first pn junction between the second region 2 and the first region 1, the first and second device terminals T.sub.1, T.sub.2 would be shorted through their portions in ohmic contact with the first region 1, making the presence of the second to fifth regions totally meaningless.
An object of the present invention is to provide a surge protection device of the breakover type that is capable of absorbing surges irrespective of the polarity thereof without need for the aforesaid ohmic electrode structure with respect to the first region, and which is capable of preventing misoperation under exposure of the device to a small surge.
Another object of the invention is to provide a surge protection device enabling highly precise control of the breakover current .+-.I.sub.BO and hold current .+-.I.sub.H preferably with increase design freedom. This object of the invention also contributes to keeping the device from responding to a surge which is small in terms of energy but nevertheless exhibits a large dV/dt.
Another object of the invention is to provide a surge protection device exhibiting a large surge absorption capacity.